1. Field of the Invention
This invention relates to a biasing/erasing oscillation circuit for magnetic tape recording apparatuses, and more particularly to a biasing/erasing oscillation circuit for VTRs which is adapted to provide an alternate current bias, for example, to a recording/reproducing head or supply to an erasing head a high frequency current for erasure.
2. Description of the Prior Art
Magnetic tape recording apparatuses, particularly VTR, are generally provided with a biasing/erasing oscillator for generating recording biases and/or erasing currents. Conventionally, the biasing/erasing oscillator of this kind uses an LC resonant circuit having a discrete inductor and capacitor connected in parallel therewith. Accordingly, the conventional oscillator is not only complicated in circuit structure but also expensive to produce.
Under such a situation, there has been disclosed an oscillation circuit for audio tape recorders by Japanese Laid-open Publication No. S61-153102 that has been laid open on Sep. 22, 1986, wherein an LC resonant circuit is configured by utilizing an inductance component of an erasing head instead of using a complicated oscillating transformer (inductor).
FIG. 1 shows a circuit diagram showing this conventional art, wherein the oscillation circuit for tape recorders is shown. The oscillation circuit is structured by an LC parallel resonant circuit formed by an erasing head EH having an inductance L1 and two capacitors C1 and C2 so that it cooperates with a 2-terminal inductor L0 to thereby control oscillating frequencies. In FIG. 1, there are further shown stereo sound recording heads RHR and RHL. These sound recording heads RHR and RHL are respectively connected to a right sound input terminal AR and a left sound input terminal AL.
The conventional art shown in FIG. 1 does not teach concrete values at all as to the resistors R1, R2 and R3 as well as the capacitors C0, C1, C2, C3 and C4, etc. for establishing required oscillating conditions. Moreover, since the conventional art is an oscillation circuit for audio tape recorders, the application of this conventional art to a VTR (VCR) requires setting on oscillating conditions necessitated for the VTR (VCR).
To cope with this, the present inventors conducted various experiments by using the FIG. 1 circuit in order to divert the conventional art of FIG. 1 to a VTR (VCR). VTR (VCR) biasing/erasing oscillators require a high frequency current of approximately 180-200 mA (RMS). Further, the impedance of the erasing head EH in the FIG. 1 circuit has been set at 80 .OMEGA., because an entire-width erasing head generally used for VTR (VCR) possesses 80 .OMEGA.. Under these propositions, the values of the inductors, capacitors, etc., for establishing an optimal oscillating condition, have been set as C0=470 pF, C1=0.18 .mu.F, C2=0.33 .mu.F, L0=220 .mu.H, R1=47 k.OMEGA. through calculations and a cut-and-try technique. Note that as a transistor TR was used "2SD734". A direct current power voltage +B of 8 V was applied to the circuit. As a result of this experiment, the transistor TR of the FIG. 1 circuit had a direct current as high as 144 mA flowing therethrough and accordingly the transistor TR was abnormally heated up where the FIG. 1 circuit is used for a VTR (VCR) biasing/erasing oscillator. It was found from the result of the experiment that the oscillator of the FIG. 1 circuit set with the above values is impossible to put into practical use.
The inventors then conducted a further experiment to apply a direct current power voltage+B of 5V to the FIG. 1 circuit set with the above respective values. In this case, the direct current flowing through the transistor TR was at 80 mA, thereby eliminating the abnormal heating up in the transistor TR. However, the erasing head EH with impedance of 80 .OMEGA. had a high frequency current as low as 140 mA (RMS). It was found in also this experiment that the FIG. 1 circuit cannot be employed as a VTR (VCR) biasing/erasing oscillator.
Another experiment was further conducted on an assumption of using two erasing heads, i.e. an entire-width erasing head and a sound erasing head. In this case, the series connection of the entire-width erasing head and the sound erasing head corresponds to the erasing head EH in the FIG. 1 circuit. In this experiment, the impedance of the entirewidth erasing head (video erasing head) was set at 80 .OMEGA. and the impedance of the sound erasing head at 34 .OMEGA. with the capacitor value, etc. set at C0=470 pF, C1=0.18 .mu.F, C2=0.027 .mu.F, L0=220 .mu.H, R1=47 k.OMEGA.. In this experiment, a direct current power voltage+B of 11 V was applied so as to allow a high frequency current of 200 mA to flow through the entire-width erasing head. The transistor TR had a direct current of as high as 280 mA flowing therethrough. Consequently, it was found also in this experiment that the conventional art of FIG. 1 is impossible to utilize for a VTR (VCR) biasing/erasing oscillator.